The leukocyte NADPH oxidase catalyzes the production of O2 from oxygen and NADPH. The yeast 2-hybrid system was used to look for proteins the interact with p67, one of the cytosolic subunits of the oxidase. This system identified a B lymphocyte cDNA that encoded a 56 kDa protein containing two Ca++ and lipid-binding C2 domains in its C-terminal half. Rabphilin-3A, a Rab3a-binding protein involved in synaptic vesicle trafficking, has a similar C2 domain structure, suggesting that the new protein, designated p56C2 and characterize it in terms of its location in neutrophils and B lymphocytes, its interactions with other leukocyte proteins and with the cortical cytoskeleton, and its association with lipids. The function of p56C2 will be studied by examining its effect on oxidase activation in the phosphorylation-dependent oxidase activating system, by examining oxidase activation and secretory vesicle fusion in cells depleted of the protein by antisense techniques, and by studying a transgenic mouse deficient in the protein. Particular attention will be paid to the possibility that p56C2 interacts with Rap1A, a small guanine nucleotide-binding protein that copurifies with the membrane- associated subunits of the oxidase. Through these experiments we hope to achieve a deeper understanding of the activation of the O2- forming oxidase of leukocytes, and enzyme important in host defense and in the harmful side effects of inflammation.